Thin film and hybrid integrated circuits have extensive use in a variety of applications. A metal interconnection system widely used for such applications includes, in an ascending order from an insulating substrate, layers of titanium, palladium and gold. Titanium provides good adhesion to the insulating substrate, palladium provides good adhesion between titanium and gold layers. Gold layer which typically is approximately 5,000 nm thick acts as a primary conductor. This metal interconnection system was superseded by a metal composite including titanium, copper, nickel and gold layers. Here, a thick layer of copper is used as the primary conductor permitting the use of a much thinner gold layer, primarily on those areas of the circuit which are intended for soldering or bonding connection. Nickel oxide, which is formed on the nickel layer, is intended to act as a barrier to solder wetting any other areas of the circuit but the gold areas on the contact pads of the circuit. This composite may optionally include a layer of palladium or palladium-titanium alloy positioned between the titanium and copper layers. For example, see U.S. Pat. No. 4, 109,297 issued Aug. 22, 1978 to N. G. Lesh et al. or U.S. Pat. No. 5,288,951 issued Feb. 22, 1994 to R. P. Frankenthal et al., both of which are incorporated herein by reference.
An example of such a prior art metallization composite is shown in crossection in FIG. 6 of the drawings. The composite includes in an ascending order from a substrate, 61, a layer of titanium, 62, a layer of palladium or palladium-titanium alloy, 63, a layer of copper, 64, a layer of nickel, 65, and a layer of gold, 66. The gold layer may extend over the whole or a part of each metallization or only over terminal pads and bonding pads of the metallizations. This interconnection system is especially useful for soldering connection to other components of IC packages, especially to bumped IC units. Bumped IC units are being used for establishing leadless solder joints to terminal pads on the board. "Leadless solder joint" means that there are no flexible conduction leads or rigid pins on the IC unit for establishing a conductive path between contact pads on the IC unit and terminal pads on the board.
However, Ni diffuses through gold and onto its surface. Nickel and its compounds, such as nickel oxide, harden the gold and promote dewetting failures of soldered joints. Therefore, it is desirable to eliminate or at least reduce diffusion of nickel onto gold surface so as to have reliable solderable connection to the bumped IC units.